1. Field of the Invention
The present invention relates to a ceramic setter plate for use in the firing of a ceramic electronic component or circuit board, and more specifically to a ceramic setter plate having excellent gas permeability, and a manufacturing method thereof.
2. Description of the Prior Art
It is common to manufacture a ceramic electronic component or circuit board by firing a laminate made by placing a plurality of ceramic green sheets on top of one another. With such a ceramic electronic component or circuit board, through the firing of the laminate the ceramic green sheets shrink, and hence the dimensions become smaller and, moreover, variations arise in the dimensions. Recently, to improve the accuracy of the dimensions after the firing, a method has thus been developed in which constraining ceramic green sheets that do not undergo sintering at the sintering temperature of the above laminated ceramic green sheets are placed on both outer surfaces of the laminate, the laminate is further sandwiched between ceramic setter plates that are placed against the outer surfaces of the respective constraining ceramic green sheets, and firing is carried out at a low temperature (1200° C. or less) while applying pressure to the laminate. With this firing method, firing shrinkage in the planar direction of the laminate can be kept down, and hence firing can be carried out with firing shrinkage in only the thickness direction.
With this firing, it is necessary to carry out binder burn-off treatment in which fresh air is fed in to decompose resin, plasticizer and solvent contained in the ceramic green sheets and the constraining ceramic green sheets, and also to discharge the decomposition gas generated. It is thus important to use ceramic setter plates that has been made to be gas-permeable; for example, an integral ceramic setter plate having cavities therein may be used and this ceramic setter plate is manufactured from two ceramic setter plate compacts, which have been prepared by subjecting a ceramic powder raw material to press working in such a manner so as to form undulations on one surface thereof, by joining the ceramic setter plate compacts together at their convex surfaces and firing them for integration. Alternatively, ceramic setter plates may be prepared by firing the ceramic setter plate compacts having undulations on one surface thereof prepared as set forth above. When the thus ceramic setter plates are used for firing a laminate of ceramic green sheets, on each side of the laminate, two fired ceramic setter plates are placed and joined together at the convex surfaces thereof so as to form cavities therebetween. Fresh air is fed in and decomposition gas is discharged through the cavities.
Moreover, regarding the ceramic green sheets, if there are variations in temperature from place to place when firing, this may cause substrate cracking or deformation, and hence it is necessary to carry out the firing while minimizing differences in temperature from place to place. Consequently, a ceramic setter plate having a high thermal conductivity is used so that heat is easily transmitted uniformly through the ceramic green sheets, or the heating and cooling rates during the firing are reduced to eliminate temperature differences.
However, with the conventional ceramic setter plates and manufacturing methods thereof described above, there are still problems to be solved as follows.
(1) With the integral ceramic setter plate in which cavities are formed by bonding two ceramic setter plate compacts, which each has undulations formed on one surface thereof, together at the convex surfaces thereof and firing for integration, the joint strength at the bonded parts is low, and hence breaking apart occurs, the lifetime is short, and repeated use is not possible. Moreover, the cost of manufacturing the ceramic setter plate is high. Furthermore, the ceramic setter plate cannot be made thin, resulting in a drop in the firing efficiency.
(2) With the ceramic setter plate in which cavities are formed by combining two ceramic setter plates, which have been made by firing two ceramic compacts, each having undulations on one surface thereof, together at the convex surfaces thereof during the firing of laminated ceramic green sheets, each of the ceramic setter plates must be thick to ensure its strength, resulting in a drop in the firing efficiency. Moreover, regarding the placing of the two ceramic setter plates together at the convex surfaces thereof, it is not possible to obtain a stable degree of parallelism, and hence it is difficult to apply pressure uniformly to the laminate for forming the electronic component or circuit board, resulting in the occurrence of defective electronic components or circuit boards that are deformed, cracked or the like.
(3) If the porosity is made high to secure the gas permeability of the ceramic setter plate, then the thermal conductivity of the ceramic setter plate will drop, and hence temperature differences will occur between the peripheral parts and the central part of the laminate during firing of the laminate and thus uniform firing will become difficult, resulting in the occurrence of defective electronic components or circuit boards that are deformed, cracked or the like. Moreover, with the method in which the occurrence of temperature differences is avoided by reducing the heating and cooling rates during the firing of the laminate, this results in a drop in the firing work efficiency. Furthermore, if the porosity is made high to secure the gas permeability of the ceramic setter plate, then the strength of the ceramic setter plate will drop, and hence the durability will drop.